The production of liquefied natural gas is a refrigeration process that reduces the mostly methane (CH4) gas to a liquid state. However, natural gas consists of a variety of gases in addition to methane. One of the gases contained in natural gas is carbon dioxide (CO2). Carbon dioxide is found in quantities around 1% in most of the natural gas infrastructure found in the United States, and in many places around the world the carbon dioxide content is much higher.
Carbon dioxide can cause problems in the process of natural gas liquefaction, as carbon dioxide has a freezing temperature that is higher than the liquefaction temperature of methane. The high freezing temperature of carbon dioxide relative to methane will result in solid carbon dioxide crystal formation as the natural gas cools. This problem makes it necessary to remove the carbon dioxide from the natural gas prior to the liquefaction process in traditional plants. The filtration equipment to separate the carbon dioxide from the natural gas prior to the liquefaction process may be large, may require significant amounts of energy to operate, and may be very expensive.
Small-scale liquefaction systems have been developed and are becoming very popular. In most cases, these small plants are simply using a scaled down version of existing liquefaction and carbon dioxide separation processes. The Idaho National Laboratory has developed an innovative small-scale liquefaction plant that eliminates the need for expensive, equipment intensive, pre-cleanup of the carbon dioxide. The carbon dioxide is processed with the natural gas stream, and during the liquefaction step the carbon dioxide is converted to a crystalline solid. The liquid/solid slurry is then transferred to a separation device that directs a clean liquid out of an overflow, and a carbon dioxide concentrated slurry out of an underflow.
The underflow slurry is then processed through a heat exchanger to sublime the carbon dioxide back into a gas. In theory, this is a very simple step. However, the interaction between the solid carbon dioxide and liquid natural gas produces conditions that are very difficult to address with standard heat exchangers. In the liquid slurry, carbon dioxide is in a pure or almost pure sub-cooled state and is not soluble in the liquid. The carbon dioxide is heavy enough to quickly settle to the bottom of most flow regimes. As the settling occurs, piping and ports of the heat exchanger can become plugged as the quantity of carbon dioxide builds. In addition to collecting in undesirable locations, the carbon dioxide has a tendency to clump together making it even more difficult to flush through the system.
The ability to sublime the carbon dioxide back into a gas is contingent on getting the solids past the liquid phase of the gas and into a warmer section of a device without collecting and clumping into a plug. As the liquid natural gas is heated, it will remain at approximately a constant temperature of about −230° F. (at 50 psig) until all the liquid has passed from a two-phase gas to a single-phase gas. The solid carbon dioxide will not begin to sublime back into a gas until the surrounding gas temperatures have reached approximately −80° F. While the solid carbon dioxide is easily transported in the liquid methane, the ability to transport the solid carbon dioxide crystals to warmer parts of the heat exchanger is substantially diminished as liquid natural gas vaporizes. At a temperature when the moving, vaporized natural gas is the only way to transport the solid carbon dioxide crystals, the crystals may begin to clump together due to the tumbling interaction with each other, leading to the aforementioned plugging.
In addition to clumping, as the crystals reach warmer areas of the heat exchanger they begin to melt or sublime. If melting occurs, the surfaces of the crystals become sticky causing the crystals to have a tendency to stick to the walls of the heat exchanger, reducing the effectiveness of the heat exchanger and creating localized fouling. The localized fouling areas may cause the heat exchanger to become occluded and eventually plug if fluid velocities cannot dislodge the fouling.
In view of the shortcomings in the art, it would be advantageous to provide a heat exchanger and associated methods that would enable the effective and efficient sublimation of solid particles therein. Additionally, it would be desirable for a heat exchanger and associated methods to be able to effectively and efficiently mix, heat, cool and/or convey fluids containing sticky, corrosive, and/or reactive chemicals.